Some electric motor/generators are referred to as permanent magnet motors. Such motors have a stator with windings carrying three-phase alternating current that creates an electromotive force to turn a rotor that has permanent magnets. At the same time, the rotating magnetic fields of the permanent magnets generate a “back electromotive force” (EMF) in the windings. The so-called back electromotive force is a voltage opposing the voltage in the stator windings. The magnetic field strength and back electromotive force acting on each phase of the stator winding varies sinusoidally with the rotational position of the rotor. The average value of the back electromotive force is proportional to the rotational speed of the rotor. At low speed conditions, the voltage produced within the windings by the back electromotive force is relatively low, and the back electromotive force will be insignificant in comparison to the input voltage. At high output speed, the voltage produced within the windings by the back electromotive force is relatively high, and significant in comparison to the maximum voltage that can be applied to the windings, so that little or no current will flow in the windings without reduction of the magnetic field of the rotor.
Typically, a field weakening current is applied to the stator windings to suppress the magnetic field and back electromotive force so that torque-producing current will flow or flow more freely through the windings at high rotor speeds. Mechanical reduction of back electromotive force, especially during high speed conditions, could significantly improve the efficiency of the motor/generator by reducing or eliminating the need for field-weakening currents through the stator windings, so that all currents flowing through the stator windings are for the production of torque and therefore useful work.